1. Field of the Invention
The invention relates to a method for operating a sheet forming unit for a machine for producing fibrous webs, in particular paper, cardboard or tissue webs from at least one fibrous stock suspension.
2. Description of the Related Art
The production process of fibrous webs is substantially dependent on the stock consistency of the fibrous stock suspension being used. With increasing stock consistency of the used fibrous stock suspension a deteriorating formation of the fibrous web at the end of the process which can be described through the macroscopic and microscopic distribution of fibers and fillers can be observed. In order to achieve satisfactory results in regard to the quality of the fibrous web, fibrous stock suspensions having stock consistencies in the range of 0.8-1.2% are brought into the downstream forming units in the current conventional headboxes. If stock consistencies with higher values are used, a coarsely clouded formation inside the fibrous stock suspension is to be expected, already at the outlet of the jet from the headbox, due to heavy fiber flocculation. Measures are therefore to be taken to facilitate destruction of these flakes and timely fixing of the flow. In particular, an as flake-free fibrous stock suspension jet as possible is to be provided through the headbox at its outlet. Inside the turbulence generating device which is arranged before the nozzle, regions serving the de-flocculation and better fluidization for the fibrous stock suspension are therefore provided by different means in turbulence generating channels. Many times these are however not sufficient. The reason is the greatly reduced re-flocculation time with increased stock consistency. However, in order to achieve satisfactory formation parameters for the developing fibrous web, re-flocculation of the fibrous stock suspension is to be completely avoided if possible in the headbox after the most recent fluidization. This however, assumes appropriately short construction of units, which again are adverse to other requirements, in particular rigidity and reduction of the vibration tendency, as well as avoidance of hydraulic disturbances.
The problem of flake formation and its effect upon the quality of the developing fibrous web is described in publication EP 1 313 912 B1. As a solution, one design of a headbox with a modified turbulence generating device is suggested, whereby inside the turbulence generating device a fluidization is undertaken only once in one step in each turbulence generating channel of the turbulence generating device, thereby causing an acceleration of the flow and short dwell time of the fibrous stock suspension in the headbox. The level of fluidization can be maintained through the special design of the lamellas of the nozzle. For the fluidization, graduated changes of the cross sectional area of the individual turbulence generating channel of the turbulence generating device and lengths of the individual partial regions of the flow channels of the turbulence generating device forming the fluidization region are suggested which result in a length of the turbulence generating device in a range of 400 mm.
To improve the formation and the tear length properties of the developing fibrous web, a multitude of additional measures are already known which are characterized through a modification of the nozzle or the turbulence generating device.
Publication DE 101 06 684 A1 discloses one embodiment of a headbox with a specially designed lamella end for avoiding instabilities in the flow inside the nozzle and thereby a stimulation of vibration, whereby the lamella end is slanted on the side facing the nozzle wall and which, on the side facing away from the nozzle wall is provided with a structure. To influence the formation it is also known from publication DE 199 02 621 A1 to design the nozzle with different geometric regions to produce different flow cross sections inside the nozzle.
Publication WO 2008/077585 A1 discloses the promotion of the development of symmetric properties in a Z-direction over symmetrically designed headbox nozzles and the embodiment and dimensioning of same.
Measures for improving the transverse rigidity through alignment of the fibers in the region of the outlet from the nozzle are described in publication EP 1 022 378 A2. Design of the nozzle includes a region having a constant cross sectional reduction and an adjacent shorter region of constant cross sectional expansion.
In order to avoid bursting of the free jet during its exit from the nozzle, document DE 297 13 433 U1 discloses one embodiment of a headbox having a nozzle formed by machine-wide limiting areas of contact whereby at least one of the limiting surfaces is characterized by at least three segments of different angles of convergence.
Document DE 102 34 559 A1 discloses one embodiment of a headbox in a sheet forming system, wherein the nozzle is characterized by a length of ≧400 mm, whereby the turbulence block, which is formed by the turbulence generating device and which is located upstream from the nozzle, preferably is also within this length range.
All already known measures are however not suitable for bringing the dwell time of the individual fibrous stock suspension below its re-flocculation time, in particular at a higher stock consistency.